This invention relates in general to electrophotographic imaging members and, more specifically, to layered photoreceptor structures with improved combination of overcoat layer and charge transport layer and processes for making and using the imaging members.
Electrophotographic imaging members, i.e. photoreceptors, typically include a photoconductive layer formed on an electrically conductive substrate. The it photoconductive layer is an insulator in the dark so that electric charges are retained on its surface. Upon exposure to light, the charge is dissipated.
Many advanced imaging systems are based on the use of small diameter photoreceptor drums. The use of small diameter drums places a premium on photoreceptor life. A major factor limiting photoreceptor life in copiers and printers, is wear. The use of small diameter drum photoreceptors exacerbates the wear problem because, for example, 3 to 10 revolutions are required to image a single letter size page. Multiple revolutions of a small diameter drum photoreceptor to reproduce a single letter size page can require up to 1 million cycles from the photoreceptor drum to obtain 100,000 prints, a desirable goal for commercial systems.
For low volume copiers and printers, bias charging rolls (BCR) are desirable because little or no ozone is produced during image cycling. However, the micro corona generated by the BCR during charging, damages the photoreceptor, resulting in rapid wear of the imaging surface, e.g., the exposed surface of the charge transport layer. For example wear rates can be as high as about 16.mu. per 100,000 imaging cycles. Similar problems are encountered with bias transfer roll (BTR) systems. One approach to achieving longer photoreceptor drum life is to form a protective overcoat on the imaging surface, e.g. the charge transporting layer of a photoreceptor. This overcoat layer must satisfy many requirements, including transporting holes, resisting image deletion, resisting wear, avoidance of perturbation of underlying layers during coating. Although various hole transporting small molecules can be used in overcoating layers, one of the toughest overcoatings discovered comprises cross linked polyamide (e.g. Luckamide) containing N,N'-diphenyl-N,N'-bis(3-hydroxyphenyl)-[1,1'-biphenyl]-4,4'-diamine. This tough overcoat is described in U.S. Pat. No. 5,368,967, the entire disclosure thereof being incorporated herein by reference.
Durable photoreceptor overcoatings containing cross linked polyamide (e.g. Luckamide) containing N,N'-diphenyl-N,N'-bis(3-hydroxyphenyl)-[1,1'-biphenyl]-4,4'-diamine (DHTBD) [Luckamide-DHTBD] have been prepared using oxalic acid and trioxane to improve photoreceptor life by at least a factor of 3 to 4. Such improvement in the bias charging roll (BCR) wear resistance involved crosslinking of Luckamide under heat treatment, e.g. 110.degree. C.-120.degree. C. for 30 minutes. However, adhesion of this overcoat to certain photoreceptor charge transport layers, containing certain polycarbonates (e.g., Z-type 300) and charge transport materials [e.g., bis-N,N-(3,4-dimethylphenyl)-N-(4-biphenyl )amine and N,N'-diphenyl-N,N'-bis(3-methylphenyl)-[1,1'-biphenyl]-4,4'-diamine] is greatly reduced under such drying conditions. On the other hand, under drying conditions of below about 110.degree. C., the overcoat adhesion to the charge transport layer was good, but the overcoat had a high rate of wear. Thus, there was an unacceptably small drying conditions window for is the overcoat to achieve the targets of both adhesion and wear rate. Another shortcoming of the prior art is the very low charge carrier mobilities in the overcoat. If the charge carrier mobility is low, the charge carriers (created during the exposure step) that have transited through the transport layer might still be in transit in the overcoat by the time the exposed region of the photoreceptor arrives at the development subsystem. This results in higher Photoinduced discharge Characteristic (PIDC) tail voltages. PIDC is the plot of the potential versus the exposure. PIDC tail is the voltage remaining on the photoreceptor at higher exposure levels. Maximum discharge is observed if the photogenerated carriers (created during the exposure step) transit the transport layer and the overcoat layer. To the extent the carriers are still in transit, lower discharge results for a given exposure. Therefore, in order to achieve maximum discharge with lower mobility material in the overcoat, the overcoat thickness has to be small. Small thickness limits the wear life of the overcoating. In order to increase life, it is necessary to reduce wear rates and increase the overcoat thickness. Thicker overcoats require higher mobilities in order to accomplish maximum discharge for a given exposure.